The invention relates to a method and an arrangement in connection with a control gear, and a control gear.
Fluorescent tubes are used to provide lighting in residential buildings, public premises and in public transportation vehicles. Fluorescent tubes provide adequate and natural lighting and in addition, the lighting is provided with low electrical power consumption.
What is generally used in connection with fluorescent tubes is electronic control gear providing high-frequency voltage for the tube. A control gear allows controlling the tube appropriately so that the controls required by the different operational stages of the tube can easily be taken into account. For example, when a fluorescent tube is turned on, a voltage should be provided between the electrodes thereof that is higher than the voltage in a conventional operating situation in order to provide higher electron emission.
A single electronic control gear may be employed to feed several fluorescent tubes. It is advantageous to use the fluorescent tubes in parallel, because in such a case each tube requires only the output side components of the control gear. The input side of the control gear typically comprises electronics for generating direct voltage from supplying voltage or otherwise forming the voltage to be suitable for the actual output part of the control gear. Components are thus saved when used in parallel and consequently space when installing the control gear.
Publication U.S. Pat. No. 6,362,575 illustrates a control gear that allows using several fluorescent tubes in parallel. The solution is based on observing the magnitude of the current in the electrodes at the other end of the tubes. The apparatus according to the publication detects a change in the number of tubes based on the observations on said current and increases the voltage to be fed to the tubes temporarily, while the number of tubes increases in order to ensure the ignition of the added tube. However, the presented solution provides such a drawback that when tubes are employed in parallel using the same voltage, the currents of the tubes deviate, which in turn results in luminous efficiencies of varying capacities between the tubes. Furthermore, the apparatus does not provide an indication of a damaged tube that needs to be changed.
It is an object of the present invention to provide a method that avoids the above drawbacks and enables to reliably employ parallel-connected fluorescent tubes. This object is achieved with the method according to the invention, characterized by comprising the steps of controlling the control gear to provide output current having a first magnitude, determining continuously the working condition of the fluorescent tubes connected to the control gear, controlling the output current of the control gear to be divided substantially evenly between both fluorescent tubes, when said tubes are in working condition, detecting a change in the working condition of one or both of the tubes to be fed, changing the output current of the control gear to a second magnitude of the output current in response to the change detected in the working condition, and controlling the control gear to provide output current having the second magnitude.
The method according to the invention is based on the idea that the working condition of fluorescent tubes is continuously under observation, and when a change is detected in the working condition of the fluorescent tubes, the control gear is used to change the magnitude of the current. In addition, in the method according to the invention, both fluorescent tubes are provided with current of the same magnitude, when the tubes are in working condition. The method according to the invention provides a distinct indication concerning the working condition of the tubes. Furthermore, since the same current is fed into both tubes, the lighting provided by the tubes does not significantly deviate from one another.
The invention also relates to an arrangement in connection with a control gear arranged to control two parallel-connected fluorescent tubes. The control gear comprises control means arranged to provide output current for the control gear in response to a current reference, whereby the control means are arranged to control the control gear to provide output current having a first magnitude. The arrangement is characterized by further comprising means for determining the working condition of the fluorescent tubes connected to the control gear, means for changing the output current of the control gear to a second magnitude of the output current in response to the change in the working condition of the fluorescent tubes, and means for dividing the output current of the control gear between the fluorescent tubes connected to the control gear, when both tubes are in working condition, and also means for controlling the division means of the output current.
Such an arrangement allows achieving the advantages, the method according to the invention offers using a simple and therefore advantageous apparatus.
The invention further relates to a control gear for controlling two parallel-connected fluorescent tubes. The control gear comprises a control part and an inverter part, whereby the control part is arranged to control the inverter part in order to provide output current of the control gear, and elements for determining the working condition of the cathodes in the fluorescent tubes connected to the control gear. The control gear is characterized by further comprising a current balancing inductor arranged to provide both fluorescent tubes with tube current of substantially the same magnitude, when both tubes are in working condition, a current measuring resistor arranged to determine the magnitude of the output current in the control gear and is connected to a control part in order to indicate the magnitude of the output current to the control part, and also a change-over switch arranged to simultaneously change the magnitude of the current measuring resistor and to switch the current balancing inductor into use or out of use in response to the elements for determining the working condition of the cathodes in the fluorescent tubes connected to the control gear.
Such a control gear enables to reliably implement the advantages the method according to the invention provides and to use a simple structure.